Algae is EXCELLENT for them. It contains lots of the nutrients they need--it's like supergreens--vitamins, minerals, and chlorophyll. Plus, it usually contains lots of little bugs and larvae, and they'll get protein and niacin from that.
I had not heard of algae that is toxic but I suspect it's like any kind of living creature--there are toxic versions and non-toxic versions. Given space, clean water, and plentiful food options, most creatures will not choose to eat toxic versions of things, so I wouldn't worry too much about it. If they are consuming it eagerly, it's unlikely that it's dangerous for them (exceptions, of course, for man-made dangers like antifreeze, which is both poisonous and apparently quite yummy--but I suspect you're not dumping antifreeze in your ponds so you're okay lol).